In the boil water type nuclear power plant, the thermal limit such as the minimum critical power ratio and maximum linear heat generation ratio which is an indicator of the soundness of fuel is operated and managed so as not to exceed the full limit. These thermal limits calculated by the power distribution based on reactor physics are made to be observable object substitute for the surface temperature of the fuel since it is difficult to measure directly.
These thermal limits are calculated by numerical analysis from distribution of a nuclear reaction cross section, and are calculated with the cycle of several minutes-1 hour by the reactor core performance calculation system with advanced operation throughput.
In advanced boiling water reactor power plant, in order to attain and adjust the target reactor power, it is automatically controlled that the drawing out/insertion operation of a control rod and the increase/decrease operation of a core flow.
Thus, it is necessary for the automatic control of the reactor power to observe the thermal limit continuously, but it is difficult because the calculation of the power distribution is required several minutes as mentioned above.
For this reason, the simple information made by an easy operation with the signal which real-time received from the sensor arranged inside the nuclear reactor is observed continuously instead of the direct observation of the thermal limit.
When the simple information arrives at a full limit which is the operation restrictions set up beforehand, the suspend instruction for automation is outputted and then the automatic control for the control rod and the core flow is suspended (for example, Patent Literature 1).